At present radio communication systems, such as mobile phone systems and radio LANs (Local Area Networks), are widely used. Radio communication systems include a PMP (Point-to-MultiPoint) communication system in which a BS (Base Station) can communicate with a plurality of subscriber stations (MSs (Mobile Stations), for example). An example of a PMP communication system is specified in IEEE802.16d and IEEE802.16e which are specifications drawn up by IEEE (Institute of Electrical and Electronics Engineers) (see, for example, The Institute of Electrical and Electronics Engineers (IEEE), “IEEE Standard for Local and Metropolitan Area Networks Part 16: Air Interface for Fixed Broadband Wireless Access Systems”, IEEE802.16-2004 and The Institute of Electrical and Electronics Engineers (IEEE), “IEEE Standard for Local and Metropolitan Area Networks Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems”, IEEE802.16e-2005).
In addition, in some radio communication systems an RS (Relay Station) can relay radio communication between a base station and a plurality of mobile stations. An example of a relay method is specified in IEEE802.16j (see, for example, The Institute of Electrical and Electronics Engineers (IEEE), “IEEE Standard for Local and Metropolitan Area Networks Part 16: Air Interface for Broadband Wireless Access Systems: Multihop Relay”, IEEE802.16j-2009). If a relay station is installed, extension of a coverage area or improvement in throughput can be expected. That is to say, there is a possibility that a mobile station outside a coverage area of a base station can perform communication via the relay station. In addition, even when a mobile station is in the coverage area of the base station, throughput may be enhanced by the use of the relay station.
For example, a base station transmits data to a mobile station via a relay station in the following way. The data is transmitted to the relay station by the use of a relay link between the base station and the relay station and is transferred to the mobile station by the use of an access link between the relay station and the mobile station. In addition, for example, a mobile station transmits data to the base station via the relay station in the following way. The data is transmitted to the relay station by the use of an access link and is transferred to the base station by the use of the relay link.
By the way, the improvement of a relay method is discussed animatedly in order to improve a communication speed or throughput further in such a radio communication system. During the discussion about IEEE802.16m, for example, the following proposal is made. Data corresponding to a plurality of mobile stations is combined into one block and is transferred by the use of a relay link (see, for example, Doo-hyun Sung, et al., “Relay addressing method in IEEE 802.16m”, IEEE C80216m-09/0438r1, 2009 Feb. 27). For example, when a base station transmits data to a plurality of mobile stations, a relay station identifies a destination of each piece of data included in a block and transfers each piece of data to the destination mobile station. In addition, for example, when a plurality of mobile stations transmit data to the base station, the relay station combines the data from the plurality of mobile stations into a block and transfers the block to the base station.
In this case, there arises a problem about a method for identifying a mobile station corresponding to each piece of data included in a block transmitted by the use of a relay link. In order to solve the problem, the method of adding an extension header to the block transmitted by the use of the relay link is proposed. The following two methods are proposed regarding information described in the extension header (see, for example, Doo-hyun Sung, et al., “Relay addressing method in IEEE 802.16m”, IEEE C80216m-09/0438r1, 2009 Feb. 27). One method is to describe identification information for identifying a mobile station which is a source or destination of each piece of data in the extension header. The other method is to describe a bit map the length of which corresponds to the number of mobile stations connected to a relay station in the extension header and to determine based on a bit whether or not there is data corresponding to each mobile station.
A media access control method by which a subscriber station outside a communication area of a base station can communicate with the base station via a relay station is devised (see, for example, Japanese Laid-open Patent Publication No. 2007-6199). Furthermore, the following radio communication method is devised. A radio terminal transmits a first ranging request message to a relay station. The relay station generates a second ranging request message to which an identifier which indicates that the ranging request is transmitted from the radio terminal is added, and transmits the second ranging request message to a base station (see, for example, Japanese Laid-open Patent Publication No. 2008-118500). In addition, the following communication control method is devised. A base station transmits to a user unit control information indicative of the amount of a radio resource used for a control channel (see, for example, Japanese Laid-open Patent Publication No. 2008-278341).
With the relay method proposed in Doo-hyun Sung, et al., “Relay addressing method in IEEE 802.16m”, IEEE C80216m-09/0438r1, 2009 Feb. 27, however, there is room for improvement in data transfer efficiency. That is to say, with the method of adding identification information for a mobile station to a block and transmitting the block, the amount of a free radio resource decreases by the amount of the identification information. For example, if each piece of identification information is represented by twelve bits and a block including data corresponding to five mobile stations is transmitted, then a radio resource corresponding to 60(=12×5) bits is used for an extension header.
Furthermore, with the method of transmitting a block including a bit map the length of which corresponds to the number of mobile stations connected to a relay station, the amount of a free radio resource also decreases with an increase in the number of mobile stations connected to the relay station. For example, it is assumed that one bit is assigned to each mobile station. If sixty mobile stations are connected to the relay station, then a radio resource corresponding to sixty bits is used for an extension header.
Accordingly, if the above extension header is transmitted, overhead is incurred. This overhead is problematic. In particular, if the amount of data, such as voice data in VoIP (Voice over Internet Protocol) communication, transmitted at a time is small, then the amount of a radio resource used for an extension header is relatively large. As a result, the above problem comes to the front.